Pressure regulating valves are used in myriad industrial and residential applications for controlling the downstream pressure of a fluid. For example, in chemical processing plants or oil refineries, pressure regulating valves are used to manipulate a flowing fluid to compensate for increases or decreases in demand, or other load disturbances, and thus keep the fluid pressure regulated. Similarly, pressure regulating valves may be used in plumbing fixtures to maintain a pre-determined pressure of fluid that automatically adjusts to variations in demand, such as anti-scald valves in showers or faucets. By controlling downstream pressure, pressure regulating valves compensate for variations in downstream demand. For example, as downstream demand increases, pressure regulating valves open to allow more fluid to flow through the pressure regulating valve, thus maintaining a relatively constant downstream pressure. On the other hand, as downstream demand decreases, pressure regulating valves close to reduce the amount of fluid flowing through the pressure regulating valve, again maintaining a relatively constant downstream pressure.
Pressure regulating valves can be categorized as either balanced or unbalanced. Unbalanced valves typically have high pressure inlet fluid on one side of the valve plug and lower pressure outlet fluid on the other side of the valve plug. Unbalanced valves suffer from an undesirable effect known as decaying inlet characteristic. The decaying inlet characteristic is a phenomenon in which an unbalanced valve experiences an unintended increase in downstream pressure as the upstream pressure decreases. This effect is undesirable as most pressure regulating valves attempt to maintain a constant downstream pressure. Decaying inlet characteristic is caused by fluid forces on the high pressure side of the valve plug attempting to move the valve plug to a closed position. As a result, the valve must have some mechanism to oppose this fluid force on the valve plug. Because the mechanism that opposes the fluid force typically has a set point, the force generated by such a mechanism is constant while the fluid force on the inlet side of the valve plug may vary (e.g., due to a decreasing supply of inlet fluid, or due to pressure variations upstream of the valve). Decaying inlet characteristic is particularly important to applications having a limited compressed fluid source, such as gas cylinders, tube trailers, or hydrils, because in such applications, there is a fixed supply of inlet fluid and thus, the inlet fluid pressure decreases as the inlet fluid supply decreases.
Unbalanced valves also suffer from damage that occurs to the valve seat. In unbalanced valves with high inlet pressures, the fluid pressure acting on large valve orifices can crush the valve seat. As a result, unbalanced valves are not ideal for high pressure, large orifice applications.
To address the decaying inlet characteristic in higher flow applications, balanced pressure regulators were developed. In the balanced pressure regulator, a portion of the upstream pressure is diverted to act on an unexposed portion of the valve plug or an unexposed portion of the valve plug moving mechanism. Thus, the valve plug is “balanced,” by not having a net effect of fluid pressure act on the valve plug (or valve plug moving mechanism). In this way, the decaying inlet characteristic is eliminated (or greatly reduced) because the fluid forces acting on valve plug (or on the valve plug moving mechanism) cancel out, resulting in a net zero force attributed to the fluid pressure. In other words, the process fluid itself generates very little, or no opening/closing forces.
In diaphragm-type pressure regulators, higher pressure fluid from an upstream or inlet side of the valve may be vented to a chamber above the valve plug to balance forces on the valve plug, similar to the balanced regulators described above. Typically, this balancing of fluid forces is accomplished by incorporating one or more vent channels or ports that extend through the valve plug (or through channels formed in the valve stem or through a sleeve adjacent to the valve stem) from the inlet side to the chamber.
A typical diaphragm-type pressure regulator is illustrated in FIG. 1. The pressure regulator 10 includes a valve body 20 having a fluid inlet 24 and a fluid outlet 22 that are fluidly connected by a passage 26. The passage 26 includes a throat or orifice 28 (forming the narrowest part of the passage 26) in which a valve seat 30 is disposed. A bonnet 32 houses a load spring 34 that is connected to a valve stem 36. The valve stem 36 is operatively attached to a valve plug 38. The valve plug 38 interacts with the valve seat 30 to control fluid flow through the valve body 20 from the inlet 24 to the outlet 22.
A diaphragm 39 is connected to the bonnet 32 and to the valve plug 38. The diaphragm 39 separates the passage 26 from a cavity 40 in the bonnet 32 that contains the load spring 34. The diaphragm 39 is responsive to pressure differences between the passage 26 and the cavity 40.
A retainer 42 is attached to the valve stem 36 and retains the valve plug 38 on the valve stem 36. The retainer may include one or more fasteners 44, such as a nut, which are attached to the valve stem 36. One or more balancing passages or channels 46 fluidly connect the passage 26 with a chamber 48 located between the valve plug 38 and the cavity 40. Fluid forces on the valve plug 38 are balanced by fluid moving through the balancing channels 46.
One problem with diaphragm-type balanced regulators, such as the balanced regulator illustrated in FIG. 1 is that they suffer from droop, or a decrease in setpoint with increased flow.